1. Field of the Invention
The present invention relates to the data compression of ECG data using delta modulation, and more particularly to an arrangement for recording ECG data in accordance with data compression utilizing a delta modulation technique.
2. Description of the Prior Art
Recent efforts in the area of cardiac technology have been directed toward the development of techniques and devices for recording ECG data derived via electrodes connected to the heart of a subject. One prior art technique or arrangement calls for the electrodes to be affixed to the patient in proximity to the heart for the development of ECG data, but such an arrangement and technique calls for the subject to be permanently connected to the recording unit. As a result, ECG data can only, as a practical matter, be developed over a relatively short period of time.
A further arrangement of the prior art is disclosed in U.S. Pat. No. 4,223,678--Langer et al, issued on Sept. 23, 1980. That patent discloses a device for recording and subsequently reproducing information, such as desired portions of an ECG signal produced by a heart, prior to and during the occurrence of a disturbance in cardiac electrical activity. Two basic embodiments of the recorder are shown: one embodiment is fully implantable and is encased within sensing and defibrillating electronics, and includes a continually updated recorder and a high-capacity stand-by recorder actuated upon the sensing of fibrillation, with the recorded information being retrieved by telemetry using an external device; the second embodiment comprises an external device having external electrodes, with the ECG information being transmitted to the recorder unit by telemetry. Reference should also be made to U.S. Pat. No. 4,295,474--Fischell, issued on Oct. 20, 1981, which shows an ECG recorder.
It should be appreciated that the above arrangements for recording ECG information are limited by the storage capacity of the recording unit. The present invention provides for a more efficient use of the limited storage capacity by employing a delta modulation data compression technique, in particular, a continuously variable slope delta modulation technique. Continuously variable slope delta modulators (CVSD) have been used, particularly in voice transmission, as described in a 1977 copyrighted publication by Harris Semiconductor, HC-55516/55532, "All-Digital Continuously Variable Slope Delta Modulator (CVSD)" and Harris Semiconductor, Application Note 607, "Delta Modulation for Voice Transmission", January, 1979.
In various prior art arrangements, such as that disclosed in the aforementioned Langer et al patent, the recording of cardiac data by fully implanted devices has necessitated the retrieval of that data by means of transmission through the skin of the subject by such means as induction coupling. Techniques such as induction coupling are characterized by disadvantages which are well known to those of skill in the art. Thus, the present invention is also directed to overcoming these disadvantages.
More specifically, the present invention employs a piezoelectric transducer technique which, besides being simpler than the induction coupling technique, permits the user to aurally monitor the heart beat.